The present invention relates to methods useful in treating subterranean formations, and more particularly, to consolidating relatively unconsolidated portions of subterranean formations and minimizing the flow back of unconsolidated particulate material (referred to collectively herein as “particulate migration”).
Hydrocarbon wells are often located in subterranean zones that contain unconsolidated particulates (e.g., formation fines) that may migrate within the subterranean formation with the oil, gas, water, and/or other fluids produced by a well penetrating the subterranean formation. As used herein, the term “unconsolidated particulate,” and derivatives thereof, includes loose particulates and particulates bonded with insufficient bond strength to withstand the forces created by the production of fluids through the formation. “Formation fine(s),” another term used herein, refers to any loose particles within the portion of the formation, including, but not limited to, formation fines, formation sand, clay particulates, coal fines, and the like.
The presence of these unconsolidated particulates in produced fluids is disadvantageous and undesirable in that the particulates may abrade pumping and other producing equipment and reduce the fluid production capabilities of producing zones. Unconsolidated subterranean zones include those that contain loose particulates and those wherein the bonded particulates have insufficient bond strength to withstand the forces produced by the production of fluids through the zones. “Zone” as used herein simply refers to a portion of the formation and does not imply a particular geological strata or composition.
One method of controlling particulates in such unconsolidated formations has been to produce fluids from the formations at low flow rates, so that the near well stability of sand bridges and the like may be substantially preserved. The collapse of such sand bridges, however, may nevertheless occur, possibly due to unintentionally high production rates and/or pressure cycling as may occur from repeated shut-ins and start ups of a well. The frequency of pressure cycling is critical to the longevity of the near well formation, especially during the depletion stage of the well when the pore pressure of the formation has already been significantly reduced.
Another method of controlling particulates in unconsolidated formations involves placing a filtration bed containing gravel (e.g., a “gravel pack”) near the well bore to present a physical barrier to the transport of unconsolidated formation fines with the production of desired fluids. Typically, such “gravel-packing operations” involve the pumping and placement of a quantity of certain particulates into the unconsolidated formation in an area adjacent to a well bore. One common type of gravel-packing operation involves placing a screen in the well bore and packing the surrounding annulus between the screen and the well bore with gravel of a specific size designed to prevent the passage of formation sand. The screen is generally a filter assembly used to retain the gravel placed during the gravel-pack operation. A wide range of sizes and screen configurations are available to suit the characteristics of the gravel-pack sand used. Similarly, a wide range of sizes of gravel is available to suit the characteristics of the unconsolidated particulates in the subterranean formation. To install the gravel pack, the gravel is carried to the formation in the form of a slurry by mixing the gravel with a treatment fluid, which is usually viscosified. Once the gravel is placed in the well bore, the viscosity of the treatment fluid may be reduced, and it is returned to the surface. The resulting structure presents a barrier to migrating sand from the formation while still permitting fluid flow.
However, the use of such gravel-packing methods may be problematic. For example, gravel packs may be time consuming and expensive to install. Due to the time and expense needed, it is sometimes desirable to place a screen without the gravel. Even in circumstances in which it is practical to place a screen without gravel, it is often difficult to determine an appropriate screen size to use as formation sands tend to have a wide distribution of grain sizes. When small quantities of sand are allowed to flow through a screen, formation erosion becomes a significant concern. As a result, the placement of gravel as well as the screen is often necessary to assure that the formation sands are controlled. Expandable sand screens have been developed and implemented in recent years. As part of the installation, an expandable sand screen may be expanded against the well bore, cased hole, or open hole for sand control purposes without the need for gravel packing. However, expandable screens may still exhibit such problems as screen erosion and screen plugging.
Another method used to control particulates in unconsolidated formations involves consolidating unconsolidated portions of subterranean producing zones into relatively hard permeable masses by applying a resin followed by a spacer fluid and then a catalyst. Such resin application may be problematic when, for example, an insufficient amount of spacer fluid is used between the application of the resin and the application of the external catalyst. In that case, the resin may come into contact with the external catalyst in the well bore itself rather than in the unconsolidated subterranean producing zone. Furthermore, there is uncertainty as to whether there is adequate contact between the resin and the catalyst. Additionally, when resin is contacted with an external catalyst an exothermic reaction occurs that may result in rapid polymerization, potentially damaging the formation by plugging the pore channels, halting pumping when the well bore is plugged with solid material, or resulting in a down hole explosion as a result of the heat of polymerization. Uniform placement of curable resin into the formations having long intervals is most desirable. However, formations often comprise a wide range of permeabilities even within a reservoir located along a well bore. As a result, completions involving resin consolidation, with conventional diversion techniques, have been applied in intervals of less than 50 feet, and more ideally, less than 30 feet. Also, using resins to consolidate long or large unconsolidated zones may not be practical due, at least in part, to the high cost of most suitable resins.
Well bores frequently contain one or more of a casing string, a liner, or a similar pipe-like apparatus. In some instances, a channel called a “perforation” may penetrate the wall of the casing string or liner. Typically, a perforated casing string or liner contains a plurality of perforations, which may resemble fissures, pores, openings, or any other regular or irregular shaped apertures. Perforations may permit fluid communication between the area to the interior of the casing string or liner with the rest of the well bore and the subterranean formation. This fluid communication may be desirable, inter alia, to allow produced fluids to flow from the subterranean formation and into the casing string or liner or to facilitate the introduction of treatment fluids to the subterranean formation through the casing string or liner.
Oftentimes, debris may accumulate in perforations that penetrate well bore casing. The term “debris” as used herein refers to undesirable deposits that may reduce the permeability of the perforations, including, but not limited to, scale, asphaltene, waxes, precipitates, drilling mud, gel residues, corrosion products, formation fines and other geological deposits, deposits from stimulation operations, and any other type of damage. One source of debris may be the migration of in situ fines mobilized during production or injection which then lodge themselves in the perforations, reducing fluid flow there through. Debris may also occur, inter alia, as a result of changes in temperature and/or pressure that may chemically and/or physically alter the substances present in the subterranean formation. In addition, chemical reactions between substances in the subterranean formation may result in debris, such as precipitates. Examples of typical precipitates that contribute to debris accumulation are solid salts, e.g., inorganic salts such as calcium or barium sulfates, calcium carbonate, and calcium/barium scales. The accumulation of debris in or around perforations is disadvantageous because the debris may restrict the flow of production fluids into the interior of the perforated casing. The accumulation of debris in the perforations may also decrease the permeability of the perforations to desirable treatment fluids that are not native to the subterranean formation.
Traditional methods of reducing the harmful effects of particulate migration and debris accumulation have included repeated remedial treatments, e.g., workover treatments, which reduce the amount of debris in the well bore and improve hydrocarbon production. Oftentimes these treatments must be performed every few months, as each treatment strictly treats existing problems and does not prevent debris from entering or forming in the well bore in the future. The disadvantages of performing relatively frequent remedial treatments include the delay in hydrocarbon production that is required while the treatments are performed and the recurring cost of the fluids used during the treatments.